In recent years, the trend toward micro-scale pattern rule has been increasing with the trend toward large-scale integration and high-speed of LSI. The trend toward a shorter wavelength of the exposure light source lies behind it. For example, it has become possible to mass-produce DRAM (dynamic random-access memory) of 64M-bit (processing dimension is 0.25 μm or less) by the wavelength shortening from mercury lamp i-line (365 nm) to KrF excimer laser (248 nm). Furthermore, in order to realize the production of DRAM's having integration degrees of 256M and 1 G or greater, a lithography using ArF excimer laser (193 nm) is getting studied seriously, in which studies on devices of 65-nm node have been made in combination with a lens of high NA (NA≧0.9). Although the use of F2 laser having a wavelength of 157 nm was considered as one candidate for production of devices of 45-nm node, application thereof was postponed because of many problems represented by a more expensive scanner, change of optical system, low etching resistance and the like. Hence, the ArF immersion lithography was proposed as an alternate to F2 lithography and is becoming introduced. In design rule of 45 nm or less, extreme ultraviolet ray (EUV) lithography has been hopefully expected.
As a resist suitable for such exposure wavelength, “chemically amplified resist material” attracts much attention. This contains a radiosensitive acid generator (hereinafter referred to as “photo-acid generator”) which generates an acid by radiation irradiation (hereinafter, referred to as “exposure”) and is a pattern-forming material that forms a pattern by making a difference in solubility in developing agent between the exposed portion and the unexposed portion through a reaction using the acid generated by the exposure as a catalyst.
By the way, photo-acid generator's characteristics desired in the chemically amplified resist material are exemplified by: an excellent transparency to radiation and excellent quantum yield at acid generation; a sufficiently high strength of acid to be generated; a sufficiently high boiling point of acid to be generated; and a moderate distance for which acid to be generated is diffused in a resist film (hereinafter referred to as “a diffusion length”).
Of these, with regard to acid strength, boiling point and diffusion length, the structure of anion moiety is important in a case of an ionic radiosensitive acid generator, while that of sulfonyl moiety is important in a case of a nonionic radiosensitive acid generator having normal sulfonyl structure and sulfonic acid ester structure. For example, in a case of a radiosensitive acid generator having the trifluoromethanesulfonyl structure, the strength of acid to be generated is sufficiently high and the resolution performance for photoresist is sufficiently high; however, the mask-dependence for photoresist is increased because of a long diffusion length of acid, which is disadvantageous. Additionally, in a case of a radiosensitive acid generator having a sulfonyl structure bonded to a large organic group, such as a 10-camphorsulfonyl structure, the boiling point of acid to be generated becomes sufficiently high and the diffusion length of acid becomes sufficiently short, so that the mask-dependence is reduced; however, the acid strength is not enough, so that the resolution performance for photoresist is not sufficient.
On the other hand, a radiosensitive acid generator having perfluoroalkylsulfonyl structure, such as perfluoro-n-octanesulfonic acid (PFOS), exhibits a sufficient acidity and a substantially moderate boiling point and diffusion length of acid, so as to have received attention particularly in recent years.
If taking environmental issues into account, however, the radiosensitive acid generator having perfluoroalkylsulfonyl structure, such as perfluoro-n-octanesulfonic acid (PFOS), has low flammability in general and is suspected of accumulation in the human body. Therefore, in a report made by ENVIRONMENTAL PROTECTION AGENCY (US) (see Non-Patent Document 1), it is proposed to restrict the use thereof. In micromachining fields, it is therefore an urgent matter to develop an alternate composition which does not have such a disadvantage and is excellent in function as the radiosensitive acid generator.